1. Field of the Invention
The present invention relates to a light emitting device drive circuit, and more particularly to a circuit for driving a light emitting device, such as a laser diode (LD) or a light emitting diode (LED), which emits light.
2. Related Art Statement
As is well known, a light emitting device, such as a laser diode or a light emitting diode, is a semiconductor device which emits light responsive to a drive current supplied by a prescribed drive circuit. The light emitting device is widely used as a light source in the fields of optical communications, printing plate production, etc. In general, the light emitting device has four physical properties as shown below by numbers from 1 to 4 (FIGS. 8 and 9). Due to such physical properties, the light emitting device has problems, such as a response delay between the supply of a drive current and the actual lighting up (FIG. 10), and a droop phenomenon caused by a temperature rise (FIG. 11).
1. The intensity of light emission (a light output Po) is increased in accordance with increase in a driving current (a forward current If).
2. Temperature rises when a current flows.
3. In the case where the current flow is constant, an operating voltage (a forward voltage Vop) drops in accordance with the temperature rise, resulting in decrease of the intensity of light emission.
4. The time (a rise time) required for transition from an extinction state to a light emission state is long.
Accordingly, in order to use the light emitting device, various contrivances are adopted for optimizing and stabilizing the intensity of light emission.
For example, in the case of a drive circuit (as illustrated in FIG. 17) for use in an optical communications apparatus for directly modulating the intensity of a laser diode, when the drive circuit applies to the laser diode a prescribed threshold current Ith for distinguishing between an extinction state and a light emission state, a bias current IB at a level of 0.9×Ith to 0.95×Ith is constantly supplied. In this manner, the bias current IB is set at as high a level as possible in the extinction state (a contrast ratio is reduced as much as possible), thereby improving the response from extinction to lighting of the laser diode (FIG. 18).
Such a technique for reducing the contrast ratio by the application of a high bias current is effective in the field of optical communications, where it is only required to determine whether the light emitting device is in the light emission state or in the extinction state. However, such a technique causes inconveniences in the field of printing plate production where the laser diode is used as a light source for exposing a photosensitive material. Specifically, the laser set at the bias current exposes portions of the photosensitive material (a printing plate, a prepress film, a direct digital color proofing (DDCP) photosensitive material, etc) which are originally not supposed to be exposed to light, and therefore the higher the bias current becomes (i.e., the contrast ratio becomes smaller), the narrower is the range of a representable image density, resulting in deterioration of prepress quality.
In the field of optical communications, it is hard to imagine the case where the light emission state of the light emitting device lasts continuously for a long period of time, and therefore no specific measures are taken against the droop phenomenon. In the field of printing plate production, however, the light emission state of the light emitting device may last continuously for a long period of time (e.g., a one-line scanning period) depending on types of image data. In such a case, even if a constant current is continuously supplied to the light emitting device, the intensity of light emission varies over time due to the droop phenomenon (FIG. 11), so that the image density becomes uneven, resulting in deterioration of prepress quality.